Ultra-high-frequency signal-translating stage



Aug. 25, 1942. w. H. ALDOUS 2,294,328

ULTRA-HIGH-FREQUENCY SIGNAL-TRANSLATING STAGE Filed Sept. 25, 1940 k .ILE

' INVENTOR WILLIAM H. ALDOUS' ATTORNEY Patented Aug. 25, 1942 Um --nron-rncousucr storms.-

rnsusmrmo smar William H. Aldous, Wcmbley, England, assigncr to Haseltine (Jorporation, a corporation of Delaware Application September 25, 1946, Serifl No. 358,323 in Great Britain November Ti, 1939 Claims.

This invention relates to ultra-high-frequency signahtranslating stages for operation at such high frequencies that conductance of the input circuit of the stage is an appreciable factor in determining the response of the stage.

The present invention is an improvement of the invention disclosed and claimed in the concluding application of Harold A. Wheeler, Serial No. 277,862, filed J1me 7, 1939, now United States Letters Patent No. 2,27tt54i3, issued April 14, 1942,

assigned to the same assignee as the present application.

The input conductance of conventional vacuum tubes employed, in ultra-high-frequency signaltranslating stages, such as ultra-high frequency amplifiers, tends materially to reduce the re spouse of the stage. At frequencies above rnegacycles or thereabout, the input conductance of conventional vacuum tubes is app While at frequencies higher than 50 megacycles, the input conductance, rather than the inherent tube and circuit capacitance, becomes the limiting factor in the response of the stage. The reason for this condition is that the maximum impedance which can be developed across the input circuit of the stage, which determines the effective coupling to the preceding stage, at such ultra-high frequency is limited by the input conductance of th tube.

Also, at ultra-high frequencieathe input capacitance of such a stage and the variation of input capacitance with variations of the transconductance of the vacuum tube employed are both appreciable, thereby adding other limitations of th transconductance of the tube when utilized in such an ultra-high-frequency stage, such arrangements in themselves have not been efiectivs to compensate for the conductance of the input circuit and some arrangements for stabilizing the input capacitance have even introduced appreciable additional conductance into the input circuit. Arrangements have been proposed for efiectively reducing to a low value the conductance of the input circuit of the stage. Such arrangements have generally been satisfactory where the anode load of the tube em-- ployed has a very small impedance at the frequency of the signals translated by the stage but anode loads of large impedance at th signal frequency tend to reduce somewhat the compensation efiected, since the anode potential may vary greatly during each cycle of the translated signal.

It is an object of the present invention, therefore, to provide an improved ultra-high-freuuency translating stage which is not subject to the above-mentioned disadvantages and limitations of the arrangements of the prior art.

It is a further object or the invention to provide an improved ultra high-frequency signaltranslating stage having a response which is not materially limited by the conductance of th in put circuit nor the value of the load impedance in the output circuit of the vacuum tube included in. the stage.

In accordance with th invention, an ultrahigh-frequency signal-translating stage comprises a vacuum tube having a cathode, an anode, and a control electrode, and includes an input circuit coupled to said cathode and said control electrode and an output circuit coupled to said anode and said cathode, the stage having incidental reaction to the input circuit tending to develop substantial conductance thereacross under normal operating conditions. The vacuum tube includes an auxiliary anod having appreciable capacitance with said control electrode and a screen electrode between the anodes to reduce capacitive currents therebetween. The

signal-translating stage is provided with a circuit coupled to the auxiliary anode and to the cathode including inductance, and means for applying a unidirectional positive potential to the auxiliary anode, the inductance being of such value with respect to said capacitance as effectively to neutralize a substantial part of the conductance oi the input circuit.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

It is well known that one of the factors limiting the repeating ratio of signal amplifiers including a conventional vacuum tube is the inherent inductance of the cathode lead which is common to the input and output circuits of the amplifier, the presence of this inductance having a degenerative effect in the input circuit simulating an input conductance. The input conductance, due

to the inherent cathode inductance is:

Viw (Ss+Ss) LK CKc (1) where,

- w=21r times the signal frequency;

Ss=the mutual conductance from control grid to anode of the tube employed;

Ss=the mutual conductance from control grid to accelerating or screen grid; and

cxc the efiective capacitance between the oathode and control grid.

A As taught by the aforementioned Wheeler application, if an inductor Is is positioned in the circuit of the accelerating or screen grid, it produces in the input circuit an apparent input conductance:

V I -W SILaCsc (2) where Cu is the capacitance between the control and screen grids. Accordingly, the effect of inherent inductance Lx of the cathode lead can be compensated by placing in the screen-grid circuit an inductor Ls having a value:

- Li: and applied to the control grid through the control grid-cathode capacitance CKC.

This method of compensation is effective in oscillator-modulators or frequency changers in which the anode load impedance has usually a relatively small value at the frequency of the input signal. In signal-translating stages such as signal amplifiers, however, the anode load impedance necessarily has a large value at the signal frequency. This causes variations of anode potential occurring during normal operation to be fed back to the screen grid through the anode screen-grid capacitance CAB, thereby causing corresponding variations in a potential of the screen grid which may detrimentally affect the compensation of the input conductance.

Referring now to the single figure of the drawing, there is represented a circuit diagram of a stage of ultra-high-frequencyamplification embodying the invention. This stage comprises a vacuum-tube amplifier II having an input circuit including an inductor I2 tuned by capacitance III. The tube I I is provided with a cathode 2 and a control grid or electrode 3; which are connected to the input circuit, and has an anode and cathode 2 connected in an output circuit having a high anode-load impedance and including an inductor I3 tuned by a condenser I4 to the frequency of the signal to be translated. The tube II additionally is provided with an auxiliary anode or additional electrode I and a screen grid or electrode 4 which are connected externally of the tube through an inductor LA and energized together through a resistor I5 from a source of unidirectional potential +S. The additional electrode I is positioned to receive, under the control of the control grid 3, electrons emitted from the cathode 2. The stage has appreciable capacitance between the auxiliary anode I and the control electrode 3 which is represented by the dotted-line condenser CA0. The screen grid 4 is returned to ground for currents of signal frequency by a by-pass condenser I8. Tube II has a suppressor gn'd I! connected directly to the cathode 2. The cathode circuit has an inherent lead inductance represented in lumped form by the inductor Lx which is the portion common to both the input and output circuits of the amplifier stage. The inherent capacitance between the control grid 3 and cathode 2 is represented by .the condenser Cxc.

In considering the operation of the signaltranslating stage just described, ultra-high-frequency carrier signals applied to the input circuit of the stage are translated by tube II and appear in amplified form in the output circuit of the stage. The signal-translating stage has an incidental reaction to the input circuit tending to develop substantial conductance thereacross under normal operating conditions which is effectively simulated by the inductance L1: in the common portion of the input and output circuits. The inductance LA which is employed is of such a value with respect to the capacitance Cxc between the auxiliary anode I and the control electrode 3 as to neutralize effectively a substantial part of the conductance of the input circuit; or, stating it differently, the inductance LA is so proportioned with respect to the simulated inductance that the voltage developed across the inductance LA and coupled to the input circuit through the capacitance CAC effectively neutralizes a substantial part of the conductance in the input circuit. The inherent inductance Ln of the cathode circuit is common to the input and output circuits and, therefore, has a degenerative effect in the input circuit. However, it will be seen that negative conductance is introduced into the input circuit of the stage due to the voltage developed across the inductor LA and fed back to the input circuit through the auxiliary anode-control grid capacitance CAc whereby the positive conductance of the input circuit produced by the inherent cathode inductance LK is wholly or partially compensated by the negative conductance effect of the inductor LA. In order to produce exact compensation, the value of the inductor LA is proportioned in accordance with the following equation derived by analogy to Equation 3:

Lar

LA LK SA CAC In this operation, it will be observed that the screen grid 4 is at ground potential for currents of signal frequency and its potential, therefore, is fixed regardless of any variations of potential of the anode 5. During operation of the amplifier stage, the screen grid 4 thus electrostatically shields the auxiliary anode I from the main anode 5, thereby reducing capacitive currents therebetween, and the potential of the auxiliary anode, therefore, varies only in accordance with the potential drop developed across the inductor LA and independently of the potential of the anode 5. Thus, the compensation of the input conductance is in no manner detrimentally affected by the anode potential variations as determined by the value of the load impedance in the output circuit of the vacuum tube. The suppressor grid I1 aids the screen grid 4 in electrostatically shielding the auxiliary anode I from the main anode 5.

The inductor LA may be either inside or outside of tube II and at ultra-high frequencies, as at frequencies of 10 megacycles or more, may be a straight piece of wire having a length of a few centimeters. Accordingly, if the cathode extends away from the tube seal and the auxiliary anode is placed at the end of the cathode remote from the seal, the lead from the auxiliary anode to the exterior of the tube through the seal may serve as the inductor LA. However, in order that the capacitance between the auxiliary anode and its lead, on the one hand, and the other electrodes of the tube and their leads, on the other hand, may be small, it may be desirable to bring the leads to the control grid and auxiliary anode out at one end of the tube'envelcpe and the leads to the other tube electrodes out at the other end of the envelope. In this event, the inductor LA tube of the type wherein the mutual conductance between the control grid and screen grid is reduced to a small value by the provision of a guiding grid, the function of which is to prevent current flowing to the screen grid. It is evident from Equation 3 that, if the control-screen-grid conductance S5 is small, an inductor placed in the screen-grid circuit in accordance with prior teachings must be so large that it may be impractical.

It may be observed that the value of the inductor LA need not necessarily provide exact compensation of the input conductance but may over-compensate to reduce the input conductance component of the stage caused by the finite transit time of the electrons. The transit time effect causes an input conductance that varies similarly with frequency and transconductance to that component caused by cathode lead inductance and evaluated by Equation 1. Thus, the efiects, both of transit time and cathode lead inductance, can be either exactly compensated or over-compensated by suitable selection of the value of LA in accordance with the invention.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in. the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is clabned is:

1. An ultra-high-irequency signal-translating stage comprising, a vacuum tube having a cathode, an anode, and a control electrode, an input circuit coupled to said cathode and said control electrode, an output circuit coupled to said anode and said cathode, said stage having incidental reaction to said input circuit tending to develop substantial conductance thereacross under normal operating conditions, an auxiliary anode in said vacuum tube, said stage having appreciable capacitance between said auxiliary anode and said control electrode, a screen electrode between said anodes to reduce capacitive currents therebetween, a circuit coupled to said auxiliary anode and said cathode including inductance, and means for applying a unidirectional positive potential to said auxiliary anode, said inductance being of such value with respect to said capacitance as effectively to neutralize a substantial part of said conductance of said input circuit.

2. An ultra-high-frequency signal-translating stage comprising, a vacuum tube having a cathode, an anode, and a control electrode, an input circuit coupled to said cathode and said control electrode, an output circuit coupled to said anode and said cathode and including a portion common to said input circuit, said stage having incidental reaction to said input circuit tending to develop substantial conductance thereacross 1 under normal operating conditions of said stage which is effectively simulated by an inductance in said common portion of said input and output circuits, an auxiliary anode in said vacuum tube, said stage having appreciable capacitance between said control electrode and said auxiliary anode, a screen electrode between said anodes to reduce capacitive currents therebetween, a circuit coupled to said auxiliary anode and said cathode and including inductance, and means for applying a unidirectional positive potential to said auxiliary anode, said inductance being so proportioned with respect to said simulated inductance that the voltage developed across said inductance and coupled to said input circuit through said capacitance effectively neutralizes a substantial part of said conductance in said input circuit.

3. An ultra-high-frequency signal-translating stage comprising, a vacuum tube having a cathode, an anode, and a control electrode, an input circuit coupled to said cathode and said control electrode, an output circuit coupled to said anode and said cathode, said stage having incidental reaction to said input circuit tending to develop substantial conductance thereacross under normal operating conditions, an additional electrode positioned to receive under control of said control electrode electrons emitted from said cathode, said stage having appreciable capacitance between said control electrode and said additional electrode, means for electrostatically shielding said additional electrode from said anode, a circuit coupled to said additional electrode and said cathode including inductance, and means for applying a unidirectional positive potential to said additional electrode, said inductance being of such value with respect to said capacitance as eifectively to neutralize a substantial part of said conductance of said input circuit.

4. An ultra-high-irequency signal-translating stage comprising, a vacuum tube having a cathode, an anode, and a control electrode, an input circuit coupled to said cathode and said control electrode, an output circuit including a high anode load impedance coupled to said anode and said cathode, said stage having incidental reaction to said input circuit tending to develop substantial conductance thereacross under normal operating conditions, an auxiliary anode in said vacuum tube, said stage having appreciable capacitance between said auxiliary anode and said control electrode, a screen electrode between said anodes to reduce capacitive currents therebetween, a circuit coupled to said auxiliary anode and said cathode including inductance,

and means for applying a unidirectional positive potential to said auxiliary anode, said inductance being of such value with respect to said capacitance as efiectively to neutralize a substantial part of said conductance of said input circuit.

5. An ultra-high-frequency signal-translating stage comprising, a vacuum tube having a cathode, an anode, an auxiliary anode, a screen electrode positioned between said anodes to reduce the capacitive currents therebetween, and a control electrode, an input circuit coupled to said cathode and said control electrode, an output circuit coupled to said anode and said cathode and including a cathode lead common to said input circuit, said stage having combined mutual conductance Sa between said control electrode and said anode and screen electrode and having incidental reaction to said input circuit tending -to develop substantial conductance thereacross under normal operating conditions of said stage, which is efiectively simulated by an inductance L1: in said cathode lead common to said input 5 anode and said cathode and including induct- I0 ance LA, and means for applying a unidirectional positive potential to said auxiliary'anode, said inductance being proportioned in accordance with the expression where Cxc is the capacitance between said grid and said cathode.

WILLIAM H. ALDOUS.

Certificate of Correction Patent No. 2,2 94,328, August 25, 1942. WILLIAM H. ALDOUS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 1, second column, line 47,

for that portion of Equation 1 reading Vi-e0 read ,V =w page 2, second column, line 37, for S, read 8,; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 6th day of October A. D. 1942.

[SEAL] HENRY VAN ARSDALE,

Acting Commissioner 0] Patents. 

